In general, a one-way clutch is equipped with an outer ring and an inner ring which rotate relative to each other, and sprags or cams are wedged between the outer ring and the inner ring with rotation in only one direction but are free when rotation is in the opposite direction.
FIG. 9 is an axial cross-sectional view of a stator S equipped with a conventional one-way clutch, and illustrates an impeller 31 of the stator S, a radially-inner cylindrical part 32 of the impeller 31, and a one-way clutch 100 arranged on an inner circumferential wall of the radially-inner cylindrical part 32. Numerals 33 and 34 designate an outer ring and inner ring of the one-way clutch 100, respectively. Both of these outer and inner rings 33, 34 are axially restrained and supported by bushes 35. These bushes 35 also act as bearing tracks for needle bearings 36 arranged on opposite sides of the one-way clutch 100, respectively.
In the one-way clutch 100 shown in FIG. 9, a plurality of sprags 108 are held in place by an outer cage 101, an inner cage 107 and a ribbon spring 109. Further, numeral 37 indicates a spline groove via which the inner ring 34 is fixedly secured on a central stationary part (not illustrated) of the stator S, and sign X-X designates a central axis.
A one-way clutch is generally required to make an adequate degree of drag torque act between an outer ring and an outer cage so that the outer ring and the outer cage are caused to rotate integrally. This is to promptly transmit a motion of the outer ring to the outer cage and also to assure movements of the sprags.
It has, therefore, been a conventional practice to obtain a predetermined degree of drag torque, for example, by forming a circumferential wall of an outer cage into T-bars to hold the outer cage on an outer ring under the spring action of the T-bars or by providing the outer cage with spring members such as drag clips.
In addition, a one-way clutch of a further type is disclosed in JP-A-8-277856. In this one-way clutch, an outer cage is constructed of annular side portions, crossbars connecting the annular side portions together, and pockets (holding holes) for the arrangement of sprags. On radially-outer sides of the crossbars and at substantially central parts of the crossbars, protrusions are formed such that a circumscribed circle of the protrusions has a diameter a little greater than the diameter of the inner circumferential wall of an outer ring. The annular side portions are provided with slits to inhibit axial movements. An adequate degree of drag torque is, therefore, produced between the protrusions and the inner circumferential wall of the outer ring.
JP-A-62-188632 [U], on the other hand, discloses a cage for a one-way clutch, said cage being capable of producing a drag torque between the cage and an outer ring. The cage is formed in a shape flexed as a whole toward the inner circumferential wall of an outer ring, preferably in an elliptical shape such that at least a part of the circumferential wall of the cage is maintained in contact with the inner circumferential wall of the outer ring to use the entire cage as a spring.
The method of forming T-bars requires to partially punch holes in a flange portion and then to bend the insides of the partial holes outwardly in a radial direction, and therefore, is costly. In addition, some of the thus-bent insides of the partial holes come into contact with an outer ring to interfere with the centering of the cage. On the other hand, the method of providing the outer cage with the drag clips leads to an increase in the number of parts, thereby developing a problem that more manufacturing steps are needed.
In the one-way clutch disclosed in JP-A-8-277856, the outer cage can still be resiliently fitted in the outer ring upon fitting the outer cage in the inner circumferential wall of the outer ring even if the diameter of the circumscribed circle of the protrusions of the outer cage is a little greater than the diameter of the inner circumferential wall of the outer ring. This approach is, however, accompanied by a problem in that, as the outer cage is inserted in the outer ring while being resiliently deformed from a true circle, the thus-inserted outer cage is reduced in roundness and can provide no even drag torque. There is another problem in that the dimensional accuracy of the pockets can be hardly retained because the slits are formed at the pockets.
The one-way clutch disclosed in JP-A-62-188632 [U], on the other hand, has a compression close to 0 and can be hardly determined whether or not it has been subjected to working into an elliptical shape. It is, therefore, difficult to control its quality. Moreover, with an elliptical shape, no even drag torque acts on the whole cage as opposed to the case of a true circle, and therefore, the elliptical shape is not preferred from the functional standpoint.